Lens systems adapted for use with a primary lens for increasing focal length have been known and used for some time in many areas of photography. Similarly, anamorphic lens systems have long been used in photography, and in cinematography, where such lenses are used to reduce the amount of film required to store a quantum of information such as a scene.
In a typical application, the focal length extender comprises a lens system adapted to be directly coupled to a primary lens system. In the case of anamorphic adapters, it is known to place the entire anamorphic adapter in front of the conventional lens system, such as shown in U.S. Pat. No. 2,940,372. A similar arrangement is shown in U.S. Pat. No. 2,752,821, and smaller devices are disclosed in U.S. Pat. Nos. 2,940,372 and 3,041,935. It is also known to place the entire adapter between the primary lens and the film plane. Focal length extenders using spherical lenses have generally been placed between the film plane and the primary lens.
In specialized applications, such as where unit magnification together with variation of the anamorphotic effect is required, it is known to place part of the supplemental lens in front of a spherical lens and part behind the spherical lens, as shown in U.S. Pat. No. 3,871,748. Another variable anamorphic lens system is disclosed in U.S. Pat. 3,428,398. Complete anamorphic lens systems are disclosed in U.S. Pat. Nos. 2,890,622, 3,990,785 and 2,915,942.
Focal length extenders for typical application have generally imposed undersirable limitations in that such supplemental lens systems intended for use on the front of conventional lens systems have been massive and unwieldy because of the necessary large size thereof to prevent loss of light or vignetting, and further off-axis optical quality has frequently been inferior because of the difficulty of balancing aberrations across a large lens. In a related manner, focal length extenders on the order of two power intended for use between the film plane and the primary lens, while smaller and lighter, result in an aperture decrease on the order of a full stop or more. In addition, such post-objective adapters which are relatively small in diameter must be manufactured to extremely close tolerances to maintain good optical quality.
Thus there has been a need for a supplemental focal length extender having good optical quality with good optical speed and low mass, and which is adaptable to anamorphic applications.